1. Field of the Invention
The present invention relates to a method and apparatus for estimating a halftone image from a binary image and, more particularly, to a method and apparatus for effectively estimating an original halftone image from a binary image displayed as a pseudo halftone image.
2. Description of the Prior Art
Most existing output devices such as display devices and printers reproduce images using binary notation, i.e., black and white. A density or concentration pattern method (brightness pattern method) and a dither method are known as ways to represent a pseudo halftone image using such an output device. These methods are both types of an area gradation method, which changes the number of dots recorded within a predetermined area (matrix).
According to the density pattern method, a portion corresponding to one pixel of an original is recorded by a plurality of dots using a threshold matrix, as shown in FIG. 41(b). According to the dither method, a portion corresponding to one pixel of an original is recorded with a single dot, as shown in FIG. 41(a). Dither data outputs are obtained as shown in FIGS. 41(a) and 41(b), respectively. These data outputs represent pseudo halftone images using black and white dots.
If a binary pseudo halftone image can be reconverted to the original halftone image (corresponding to the input data in FIGS. 16(A) to 16(D)), various types of data processing can be performed to provide versatility in image conversion. In the case of the density pattern image, the original halftone image can be immediately obtained if pattern level distribution is known. However, density pattern images suffer from low resolution despite requiring a large amount of data. On the other hand, dither images have higher resolution but cannot be easily reconverted to original halftone images. Therefore, various types of image conversion cannot be performed using only dither images.